In summary, the calculator incorporates a reduction of 25% in CO2 emissions from the best "green" electricity tariffs.

A reduction of only 25% puzzles many people who read the claims of electricity companies that they are supplying 100% renewable electricity, and we have had much correspondence on the subject.

The key point is that whatever tariff you are on, when you switch on an appliance, for example an electric heater, the consequence is that a fossil fuel power station burns a bit more fossil fuel to provide that extra electricity, and so releases extra CO2 to the atmosphere. There is no unused capacity in renewable energy facilities just waiting until someone chooses a "green" tariff, and switches on an appliance. The situation is thus fundamentally different to, for example, Fair Trade bananas, where there are two separate stocks of bananas, and separate supply lines, and individual purchasing decisions can lead directly back to more Fair Trade bananas being grown.

The problem is that in the UK, the regulation of electricity generated from renewable sources is complex - every electricity supplier is required to either buy some electricity from renewable sources, or to buy certificates from other suppliers who have bought from renewable sources, or to pay a fee. This system is designed to encourage investment in renewable sources, but it results in the trading of certificates that is almost impossible for the ordinary consumer to understand, since the "greenness" of electricity can be sold to two or even three different customers by some suppliers.

Accreditation

The National Consumer Council wrote a guide [39], and concluded that many suppliers offering green tariffs were doing little more than meeting legal requirements, and were not delivering the environmental benefits that they claimed. The guide said that even the best tariffs reduced CO2 emissions by only a faction.

In response to the criticisms, Ofgem has set up an accreditation scheme (http://www.greenenergyscheme.org/). The scheme website says that using a certified tariff "helps to reduce the environmental impact associated with your home or business energy use"; creates "additional environmental benefits that wouldn't have happened without your support"; and supports investment in renewable energy in the UK. The FAQ section of the scheme website deals with the question: "Is it possible to get 100% Green Electricity through the Scheme?" The answer given is

Your supplier may have contracted with renewable generators to buy volumes of electricity that match your use. However, the electricity coming out of your socket is the same, and is not altered by moving to a green tariff. Similarly, the carbon emissions that physically result from your electricity use will not be reduced by switching tariffs - they will just be notionally reassigned to other customers. Ofgem therefore thinks that claims stating a customer will consume 100% renewable electricity, through a green tariff, are misleading. (website viewed 12.12.13)

Recommendations

The calculator therefore assumes the reduction in CO2 emissions from the best "green" tariffs to be just 25% - partly from a reduction in CO2 emissions, and partly from the influence on Government policy arising from the demonstration to the Government that some consumers are willing to pay a premium to reduce their CO2 emissions. The best tariffs are defined according to those ranking highly in the table produced by Ethical Consumer magazine (July/August 2013) (www.ethicalconsumer.org) i.e. • Good Energy • Equipower • Green Energy Dark Green, and Pale Green • LoCO2 Energy Planet, Energy Pocket+, and Energy Pocket • Ecotricity New Energy +, and New Energy • Ovo Green Energy and New Energy

It does not seem possible currently to pay for a tariff giving electricity which is "CO2-free", and while we can make an allowance for switching to a "green" tariff, we should still aim to reduce our electricity use if we want to reduce our carbon footprints.